Although stimulants are an effective treatment for Attention Deficit Hyperactivity Disorder (ADHD) in childhood, ADHD patients have serious attentional and conduct problems in adolescence even if they had stimulants in childhood. Thus, we investigated whether stimulant therapy was also effective for ADHD adolescents, with a view towards determining whether maintenance therapy should be considered for them. A double-blind study with 48 ADHD adolescents not previously treated with stimulants showed significant improvements under methylphenidate (MPD) vs. placebo with respect to parent and teacher ratings as well as performance and cognitive event-related potentials (ERPs) in a Continuous Performance Test. The cognitive changes were of similar magnitude to results with younger children as well as normal persons treated with stimulants. However, the clinical ratings revealed decreased efficacy than previously obtained with younger patients. Although it is possible that stimulant treatment is indeed less efficacious for adolescents, we hypothesize that the smaller magnitude of clinical response is only apparent and results from the fact that adolescents spend relatively little time at home or with any one teacher. Thus, parents and teachers need more time to notice treatment effects. Therefore, we propose to test 40 ADHD adolescents (12.5 - 16 years old, Tanner Stages II - IV) in a double-blind trial of 3 weeks of MPD and 3 weeks of placebo. We will obtain weekly teacher, parent, and self reports of clinical functioning as well as cognitive evaluations (performance and ERPs in Continuous Performance and Paired Associate Learning tests as well as an Arithmetic Test) at the end of each phase. Next, patients will be followed up for 3 months on their clinical dose of NPD with monthly clinical evaluations by teachers, parents, and patients. At this point, there will be a second double blind trial consisting of 2 weeks each of placebo and MFD. Clinical and cognitive evaluations will be performed in this trial as well. We hypothesize that greater benefits of stimulant therapy will be obtained in the second trial than in the first. This research will provide much needed evidence on the long-term efficacy of stimulants for ADHD adolescents. This work will also yield information that may prevent the premature discarding of an effective approach for a population at great risk for continued difficulties.